Sensor probe and shield assembly for swirl-type flowmeter

ABSTRACT

A swirl-type flowmeter whose output frequency is a function of fluid-flow rate. The inlet section of the meter flow tube is provided with a set of fixed swirl blades which impart a swirling motion to incoming fluid, the swirling fluid being caused to precess in an enlarged section of the tube. The precessional motion is detected by a thermistor probe and shield assembly to produce voltage pulses whose frequency depends on flow rate. This assembly is constituted by a probe extending transversely into the tube and supporting a bead-type thermistor at a point adjacent to the inner wall of the tube. Surrounding the probe and the thermistor is the upper end portion of an elongated tubular shield whose lower end portion has an access port therein which is oriented to capture a small sample of the precessing fluid and to conduct it in the direction of the thermistor to a discharge port beyond the thermistor, whereby the thermistor is responsive to the precessional motion but is otherwise effectively isolated from the mass of fluid flowing through the tube and from destructive particles borne by the fluid.

United States Patent [191 Burgess 1 March 6, 1973 SENSOR PROBE ANDSHIELD ASSEMBLY FOR SWIRL-TYPE FLOWMETER [75] Inventor: Thomas H.Burgess, l-lorsham, Pa.

[73] Assignee: Fischer 8: Porter Co., Warminster,

[22] Filed: June 7,1971

[21] Appl. No.: 150,278

UNITED STATES PATENTS Chanaud ..73/l94 Martino ..73/204 PrimaryExaminer-Charles A. Ruehl Assistant Examiner-Herbert GoldsteinAttorney-Michael Ebert [57] ABSTRACT A swirl-type flowmeter whose outputfrequency is a function of fluid-flow rate. The inlet section of themeter flow tube is provided with a set of fixed swirl blades whichimpart a swirling motion to incoming fluid, the swirling fluid beingcaused to precess in an enlarged section of the tube. The precessionalmotion is detected by a thermistor probe and shield assembly to producevoltage pulses whose frequency depends on flow rate. This assembly isconstituted by a probe extending transversely into the tube andsupporting a bead-type thermistor at a point adjacent to the inner wallof the tube. Surrounding the probe and the thermistor is the upper endportion of an elongated tubular shield whose lower end portion has anaccess port therein which is oriented to capture a small sample of theprecessing fluid and to conduct it in the direction of the thermistor toa discharge port beyond the thermistor, whereby the thermistor isresponsive to the precessional motion but is otherwise effectivelyisolated from the mass of fluid flowing through the tube and fromdestructive particles borne by the fluid.

7 Claims, 4 Drawing Figures SENSOR PROBE AND SHIELD ASSEMBLY FORSWIRL-TYPE FLOWMETER BACKGROUND OF THE INVENTION This invention relatesgenerally to swirl-type flowmeters, and in particular to an improvedsensor probe assembly for a flowmeter of this type, the assemblyincluding a shield to protect the sensor element and to enhance thesignal to noise ratio of the system.

A new type of volumetric flowmeter is disclosed in the article of Rodelyet al., entitled, A Digital Flowmeter Without Moving Parts, published in1965 by the American Society of Mechanical Engineers (8 Apr. 1965WA/FM6). This swirl-type flowmeter currently being manufactured by theFischer and Porter Company of Wanninster, Pa. under the trademark,Swirlmeter, and is described in their Instruction Bulletin 10Sl000.Meters of this type are also described in US. Pat. Nos. 3,279,251;3,314,289, and Re. 26,410, among others.

In a Swirlmeter, a homogeneous fluid whose flow rate is to be measured,is forced to assume a swirl component. This is accomplished by feedingthe fluid into the inlet section of a flow tube having a fixed set ofswirl blades therein which imparts a swirling motion to the fluidpassing therethrough. Downstream of the swirl blades in the tube is aVenturi section which first constricts and then expands the flow passageto transform the swirling motion into precessional movement in theexpanding region of the Venturi section to create a vortex.

Precession takes place about the central axis of the flow tube at adiscrete frequency that is a function of the volumetric flow rate.De-swirl blades in the outlet section of the flow tube serve tostraighten out the fluid leaving the meter. Cyclic variations in localfluid velocity occurring by reason of precession, are detected toprovide electrical pulses whose frequency is measured to provide anindication of flow rate.

In commercially available Swirlmeters, detection of the cyclicvariations is effected by means of a sensor probe mounted in the body ofthe meter transversely with respect to the longitudinal axis of themeter in the area where the vortex precession is near the inner wall ofthe flow tube. At the tip of the sensor probe is a thermistor whichserves to detect the frequency of precession. A thermistor is asolid-state device made of a semi-conducting oxidic material thatexhibits a high negative temperature coefficient of resistivity wherebythe resistance of the device increases rapidly with decreasingtemperature.

In the Swirlmeter, the thermistor is caused to operate in its self-heatregion by applying a constant current thereto to heat'the thermistor toa temperature above that of the fluid flowing through the meter. For agiven fluid velocity, the thermistor is caused by the fluid passingthereby to undergo an appreciable increase in resistance by means of thecooling effect produced by the fluid stream.

Inasmuch as the current applied to the thermistor is maintainedconstant, an increase in its resistance will be effective as an increasein voltage. Any increase in velocity such as that produced by a fluidicvortex will further cool the thermistor, giving rise to a furtherincrease in voltage. The voltage variations developed in the thermistorcircuit as a result of the cyclic variations in local fluid velocity,have a frequency depending on flow rate and constitute the outputsignal.

The thermistor employed in the sensor probe is of the glass-coated beadtype. Since the probe is extended into the meter pipe transversely withrespect to the longitudinal axis thereof, the fluid impinging on theexposed glass bead subjects it to bending stresses, and

since the glass bead is inherently brittle, the bead may be damagedthereby.

Moreover, in many cases, the fluid being measured contains dirtparticles and other abrasive contaminants which impinge on the exposedsurface of the glass bead and gradually erode the surface as a result ofwhich the glass bead may fracture after a relatively short period ofoperation.

I-Ience even though the Swirlmeter has no moving parts that are subjectto wear, its effective life is limited by the life of the thermistorsensor, and should the sensor fail, the meter will be renderedinoperative.

BRIEF DESCRIPTION OF THE INVENTION In view of the foregoing, it is themain object of the invention to provide a sensor probe and shieldassembly for a swirl-type flowmeter which is adapted to capture a sampleof the precessing fluid passing through the meter tube and to conduct itto the sensor to render the sensor responsive to the precessionalmotion, but to otherwise isolate the sensor from the mass of fluidflowing through the meter tube.

More particularly, it is an object of the invention to provide anassembly of the above type in which the sensor is constituted by aglass-coated, bead-type thermistor whose exposed glass surface isprotected from particles borne by the fluid conducted through the metertube and from destructive mechanical stresses, whereby the effectivelife of the thermistor is prolonged.

Also an object of the invention is to provide a thermistor probe andshield assembly whose sensitivity to the signal component of the fluidicflow is enhanced, thereby improving the signal to noise ratio of theSwirlmeter system.

Briefly stated the objects are attained in a swirl-type flowmeter havinga flow tube whose inlet section incorporates a set of swirl blades toimpart a swirling motion to incoming fluid, which swirling flow iscaused to precess in an enlarged section of the tube. The precessionalmotion is detected by a thermistor probe and shield assembly to producevoltage pulses whose frequency depends on flow rate, the assemblyincluding a probe extending transversely into the tube and supporting aglass-coated, bead-type thermistor at a point adjacent the inner wall ofthe tube.

Surrounding the probe and the thermistor is an end' portion of anelongated tubular shield whose other end portion hasan access porttherein oriented to capture a small sample of the precesssing fluid andto conduct it in the direction of the thermistor to a discharge portbeyond the thermistor, whereby the thermistor is responsive toprecessional motion but is otherwise effectively isolated from the massof fluid flowing through the tube and from destructive particles bornethereby.

OUTLINE OF THE DRAWING For a better understanding of the invention aswell as other objects and further features thereof, reference is made tothe following detailed description to be read in conjunction with theaccompanying drawing, wherein:

FIG. 1 is a longitudinal section taken through a swirltype meterincorporating a thermistor probe and shield assembly in accordance withthe invention;

FIG. 2 is an exploded perspective view of the top of the meter;

FIG. 3 schematically shows the assembly installed in the meter; and

FIG. 4 is an enlarged sectional view of the assembly.

DESCRIPTION OF THE INVENTION Referring now to FIGS. 1 and 2, aSwirlmeter which includes a thermistor probe and shield assembly inaccordance with the invention, comprises a meter body constituted by aflow conduit or tube having mounting flanges 11 and 12 at either endthereof to facilitate the interposition of the meter in a line whosefluid is to be metered. The fluid may be any homogeneous fluid or gas.

Flow tube 10 is constituted by a cylindrical inlet section 10A, acylindrical outlet section 10B, and a Venturi section 10Cinterconnecting the inlet and outlet sections. Venturi section 10C isformed with a throat entrance region of decreasing cross-sectional areaextending downstream from inlet section 10A to a constricted or throatregion which leads to a flaring or throat exit region of increasingcross-sectional area communicating with outlet section 10B. Thus theVenturi serves to constrict the flow coming from the inlet section andto expand flow in advance of the outlet section.

Mounted within inlet section 10A is a fixed array of curved swirl blades13 radiating from a hub 14. To provide a strong signal and a highinformation rate, the curvature of the swirl blades is made such as tosignificantly deflect the incoming flow of fluid with respect to thelongitudinal axis of flow tube 10.

Thus fluid entering inlet section 10A is forced by the array of swirlblades to assume a swirling motion, the swirling fluid being directedinto Venturi section 10C. In the flaring exit region of the Venturiwhere the increasing area gives rise to fluid expansion, the swirlingflow is converted into precessional motion about the longitudinal axisof the tube. The frequency of precession depends on the volumetric flowrate. To straighten out the flow leaving the meter, a set of planarde-swirl blades 15 extending radially from ahub 16, is fixedly disposedin the outlet section 10B.

Cyclic variations in local fluid velocity, as a result of theprecessional motion, are detected by a sensor probe and shield assemblygenerally indicated by numeral 17 and constituted by a probe 18 whichextends into Venturi section 10C at right angles to the longitudinalaxis thereof and terminates in a bead-type thermistor 19 whoseresistance value is varied periodically in response to temperaturechanges. produced by the precessing fluid.

Voltage pulses are derived from the thermistor through wires passingthrough probe 18, these pulses being amplified and filtered, and beingconverted into square-wave pulses of constant amplitude. The frequencyof the square-wave pulses may be measured by a conventional electroniccounter, or the signals generated by the meter may be processed by otherdigital readout or control instrumentation.

The electronic circuits operating in conjunction with sensor probeassembly 17 are contained in a casing 20 mounted on the meter body. Theelectronic circuits are not the concern of the present invention, andare mentioned only in order to explain the behavior of the Swirlmeterand how flow is converted into digital values.

Fitting over and protectively surrounding the probe and thermistor isthe upper end portion of an elongated tubular shield 21 whose lower endis closed and is provided with an axially projecting nipple 22. Nipple22 is received in an indentation 23 formed in the wall of Venturisection 10C at a point diametrically opposed to the bore in the tubethrough which the probe is inserted. The purpose of the indentation andnipple combination is physically to stabilize the shield in order toprevent flutter of the assembly when subjected to the precessing fluidicstream. A similar result is achievable by passing a'bolt through thewall of the tube, which bolt is threadably received in the lower end ofthe shield.

The probe, thermistor and shield assembly is held in place within themeter body by means of a flanged retainer ring 24 which is fastened ontoa cylindrical boss 25 formed on the meter tube by suitable screws. ABelleville washer 26 is interposed between retainer ring 24 and boss 25to maintain loading of the assembly over a wide temperature range.

Formed in the lower end section of the shield at a position displacedabout degrees from the direction of flow down the tube, is an accessport 27 which in practice, may be in round or in slot form or any othersuitable forms. Access port 27 functions to admit a sample of theprecessing fluid into the shield to produce an oscillating sample streamwhich flows upwardly in the shield in the direction of thermistor l9 andtherebeyond for exhaust through a discharge port 28 formed in the upperportion of the shield. Discharge port 28 is oriented to exhaust thesample stream in the downstream direction.

Placed within shield 21 at a divergent point therein in the upward pathof the sample stream in advance of thermistor 19, is a constriction orbaffle 29 which is adapted to focus and concentrate the sample streamonto the effective surface of the thermistor to maximize the effectthereof.

Thus the thermistor is no longer a standard arrangement, exposed to thefull brunt of the main stream passing through the meter, but only to anoscillating sample stream. In this way, the thermistor is protected fromabrasive contaminants in the fluid and from the substantial bendingstresses produced by the fluid.

Because the velocity of the sampling stream impinging on the thermistoris far less than that of the main stream, the resultant reduction in themagnitude of fluid forces imposed on the thermistor obviates damagethereto by reason of these forces. Moreover, the direction of the samplestream relative to the thermistor, as against the direction of the mainstream in the conventional arrangement is such as to produce compressiverather than shearing stresses in the glasscovered thermistor. Sinceglass has high strength with respect to compressive forces imposedthereon, the glass is not damaged in any way by the sample stream.

The present arrangement, since it extracts from the main stream anoscillatory sample stream, emphasizes the signal component and playsdown all other voltageproducing components generated in the main stream,thereby improving the signal-to-noise ratio of the system. Moreoverbecause the sample stream is focused onto the thermistor, there iseffected a marked reduction in hydraulic noise due to flow separationover the thermistor glass.

It must be borne in mind that in a swirlmeter, the precessing fluidcreates cyclic variations in local transport properties, such asvelocity, pressure, temperature or the like. The shield, in accordancewith the invention, constitutes a passage extending transversely withrespect to the longitudinal axis of the meter conduit, which passageprovides a connection between points located on either side of thisaxis, so that cyclic variations are alternately present at each end ofthe passage. The sensor disposed in the shield or passage need not,therefore, be a thermistor, but may be a fluidic amplifier to amplifythe fluidic variations present within the passage, or it may be apressure transducer or any other sensor responsive to a varying propertyencountered in the passage.

While there has been shown and described a preferred embodiment of asensor probe and shield assembly for a swirl-type flowmeter inaccordance with the invention, it will be appreciated that many changesand modifications may be made therein without, however, departing fromthe essential spirit of the inven tion.

I claim:

1. In a swirl-type flowmeter provided with a flow conduit having meansdisposed in the inlet section thereof to impart a swirling motion tofluid admitted therein, the swirling fluid being caused to precess in anenlarged section of the conduit to create cyclic variations in localtransport properties, such as variations in velocity or pressure, a tubedisposed in said conduit to define a relatively narrow passage extendingtransversely with respect to the longitudinal axis of the conduit, saidtube communicating with points in said conduit located on opposite sidesof said axis at which cyclic variations are alternately present, and asensor disposed within said tube at a position intermediate said points,said sensor being adapted to amplify or transduce said variations.

2. In a swirl-type flowmeter provided with a flow tube having swirlblades disposed in the inlet section thereof to impart a swirling motionto fluid admitted thereto, the swirling fluid being caused to precess inan enlarged section of the tube to create cyclic variations in localfluid velocity, temperature and pressure; a sensor assembly disposed insaid tube to amplify the fluidic variations or convert them intoelectrical variations, said assembly comprising:

A. a probe mounted on the wall of said tube and projecting transverselytherein,

B. a sensor mounted on the free end of the probe,

C. ai n elongated tubular shield, transversely disposed in said tube,the upper end portion of said shield enclosing both said probe and saidsensor, said shield having ports both above and below the sensor locatedso that the passage of the vortex core around the circumference of thetube creates an alternating pressure at said ports and a cor respondingoscillating flow in said shield, whereby said sensor is actuated.

3. An assembly as set forth in claim 2, wherein said sensor is aglass-coated, bead-type thermistor.

4. An assembly as set forth in claim 3, further including a baffledisposed in said shield to focus the sample stream on the thermistor.

5. An assembly as set forth in claim 2, wherein the axes of said portsare at to 270 with respect to the mean velocity direction.

6. An assembly as set forth in claim 2, further including meansattachable to the lower end of said shield to stabilize same.

7. An assembly as set forth in claim 6, wherein said stabilizing meansis constituted by a nipple attached to the lower end of the shield andreceived in an indentation in the wall of the tube.

1. In a swirl-type flowmeter provided with a flow conduit having meansdisposed in the inlet section thereof to impart a swirling motion tofluid admitted therein, the swirling fluid being caused to precess in anenlarged section of the conduit to create cyclic variations in localtransport properties, such as variations in velocity or pressure, a tubedisposed in said conduit to define a relatively narrow passage extendingtransversely with respect to the longitudinal axis of the conduit, saidtube communicating with points in said conduit located on opposite sidesof said axis at which cyclic variations are alternately present, and asensor disposed within said tube at a position intermediate said points,said sensor being adapted to amplify or transduce said variations.
 1. Ina swirl-type flowmeter provided with a flow conduit having meansdisposed in the inlet section thereof to impart a swirling motion tofluid admitted therein, the swirling fluid being caused to precess in anenlarged section of the conduit to create cyclic variations in localtransport properties, such as variations in velocity or pressure, a tubedisposed in said conduit to define a relatively narrow passage extendingtransversely with respect to the longitudinal axis of the conduit, saidtube communicating with points in said conduit located on opposite sidesof said axis at which cyclic variations are alternately present, and asensor disposed within said tube at a position intermediate said points,said sensor being adapted to amplify or transduce said variations.
 2. Ina swirl-type flowmeter provided with a flow tube having swirl bladesdisposed in the inlet section thereof to impart a swirling motion tofluid admitted thereto, the swirling fluid being caused to precess in anenlarged section of the tube to create cyclic variations in local fluidvelocity, temperature and pressure; a sensor assembly disposed in saidtube to amplify the fluidic variations or convert them into electricalvariations, said assembly comprising: A. a probe mounted on the wall ofsaid tube and projecting transversely therein, B. a sensor mounted onthe free end of the probe, and C. an elongated tubular shield,transversely disposed in said tube, the upper end portion of said shieldenclosing both said probe and said sensor, said shield having ports bothabove and below the sensor located so that the passage of the vortexcore around the circumference of the tube creates an alternatingpressure at said ports and a corresponding oscillating flow in saidshield, whereby said sensor is actuated.
 3. An assembly as set forth inclaim 2, wherein said sensor is a glass-coated, bead-type thermistor. 4.An assembly as set forth in claim 3, further including a baffle disposedin said shield to focus the sample stream on the thermistor.
 5. Anassembly as set forth in claim 2, wherein the axes of said ports are at90* to 270* with respect to the mean velocity direction.
 6. An assemblyas set forth in claim 2, further including means attachable to the lowerend of said shield to stabilize same.